Animating Points on Map

i’ve created an animated map with points on it, which cycles through several years. The points are mapped so that their size is proportional to one of my other columns. The animation works, but I would like the transition between years (which grows the size of the points) to be smooth. Currently, the code below produces an animation that ‘jumps’ between point sizes, but I would like the transition to be smooth.

opt <- list(
  scope = 'usa',
  projection = list(type = 'albers usa'),
  showland = TRUE,
  showlakes = TRUE,
  countrywidth = 0.5,
  subunitwidth = 0.5

plot_geo(df) %>%
  add_markers(x = ~long,
              y = ~lat,
              frame = ~year,
              ids = ~id,
              size = ~size) %>%
  layout(geo = opt, easing = "linear")

Am I doing something incorrectly here? Thanks so much.